F. Scott Fitzgerald lived during the Roaring Twenties. He had firsthand experience with what that time and place in American history felt like. He placed The Great Gatsby right in the middle of that time period. He is familiar with it, and his readers would be familiar with it as well. It may be a work of fiction, but that doesn't make it less representative of that era's zeigeist.
The economy was good during the 1920s. People were making money and had money to spend. Granted, a lot of that money was being made through less than legal avenues thanks to prohibition, but that doesn't mean money, wealth, and status were any less important.
One solid indicator of wealth and a booming economy is West Egg itself. West Egg is where all of the "new money" lives, including people like Gatsby. East Egg, by contrast, is the "old money." East Egg is comprised of people that have been born into their wealth. The families have been wealthy for generations and will likely be wealthy for generations more to come. The West Eggers are the new rich in town. Their wealth could be from a variety of sources. It could be because they are doing well in the stock market. Nick is likely in this camp. Not much info is given on Nick's job, but the reader is told early on that he is in the bonds business. Perhaps the money is coming from being in some new emerging market. William Randolph Hearst or John D. Rockefeller acquired their wealth this way. But more than likely, the new money was being made from bootlegging. That's how Gatsby made his money. It's how Arnold Rothstein, Al Capone, and Lucky Luciano made millions too.
In many ways, however, West Egg and East Egg really aren't that different. They have loads of money and will continue to have loads of it. The East Eggers might turn their nose up at the West Eggers and talk badly about the other group at times, but they both consider themselves "in the rich club" and hang out together, if it means parties and alcohol. That's precisely what Gatsby uses to his advantage. He needs to somehow find a way to spend more time with Daisy. He's West Egg, she's East Egg (do you see Romeo and Juliet or West Side Story yet?). The two aren't supposed to fraternize, but oh do they, when Gatsby throws a party. Daisy may say that the parties are decadent and uncivilized, but she is still there.
Gatsby's parties themselves are indicative of the wealth of the time. The entire thing is an open bar (at a time in history when alcohol is illegal); great cars, fancy dresses, etc. are all there. My favorite part of the entire book that indicates the wealth of Gatsby is when some drunk guy tells Nick that he is surprised to learn that Gatsby's library is filled with real books instead of fake ones.
It is important to note that not all of society was booming with wealth and an economy that practically printed its own money. There were still plenty of people struggling to make ends meet. Fitzgeral shows that aspect of society with the Valley of Ashes. This is where Myrtle lives and anybody from West Egg or East Egg must travel through it in order to get to the other Egg. The name itself connotes death, destruction, and stagnation. Characters like Tom abuse these people too, as indicated by how he treats George Wilson. Not only is Tom sleeping with his wife, but Tom forces George to act a certain way if he wants Tom's business. Those are hard parts of the novel to read. The entire Valley of Ashes shows a great polarity to the wealth and opulence of the Eggs. It also shows that the booming economy has its cracks and may someday come to a crashing halt. Which it does on October 29, 1929.